Method of recordng a hologram

ABSTRACT

A method of recording a hologram on a recording medium composed of a substrate and a recording material coated as a thin layer on the substrate, the recording medium being transparent or semitransparent with respect to light to which the recording medium is exposed. A value B, B/A, B/Bo, A/B, or Bo/B is established where A is the intensity of the light at a surface of the recording medium, B is the intensity of the light that has passed through the recording medium, and Bo is the intensity of the light that has passed through the recording medium immediately after the recording medium has started to be exposed to the light. The recording medium is exposed to the light until the established value is equalized to a preset value. The exposure of the recording medium is stopped when the established value is equalized to the preset value.

BACKGROUND OF THE INVENTION

The present invention relates to a method of recording a hologram.

Holograms are recently finding more use as optical elements such as hologram lenses, hologram scanners and the like in various optical devices. As optical elements, holograms are required to be optically equivalent to each other. As regards hologram disks used in hologram scanners, they must be optically equivalent to each other for compatibility, and holograms on one hologram disk must also be optically equivalent to each other.

Holograms have heretofore been recorded or photographed under fixed exposure conditions.

Holograms are recorded on a recording medium comprising a recording material coated as a thin layer on a substrate of glass or plastics. In order to be optically equivalent, holograms are made on the same recording material. However, the properties of the recording material used vary slightly as the environment changes from time to time. The manner in which the recording material is processed by exposure to light varies dependent on the condition in which the recording material has been coated, the surface condition of the substrate, the thickness of the recording material layer, and other factors. If the exposure conditions should be fixed, the diffraction efficiencies of the recorded holograms would tend to be different due to recording material irregularities such as changes in the material properties, irregular thicknesses of the coated layer, and the like.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method capable of recording optically equivalent holograms irrespective of possible recording material irregularities such as different material properties.

A recording medium on which holograms are to be recorded or photographed comprises a substrate and a recording material coated as a thin layer on the substrate. The recording medium as a whole must be transparent or semitransparent with respect to light to which it is exposed.

It is assumed that the intensity of light to which the recording medium is exposed is expressed as A at the surface of the recording medium, the intensity of such exposure light after it has passed through the recording medium is expressed as B, and the intensity of the exposure light that has passed through the recording medium immediately after the exposure of the recording medium has started is expressed as Bo. The recording medium is exposed to the exposure light until the value of B, B/A or A/B, or B/Bo or Bo/B becomes equal to a preset value. Upon coincidence of these values, the exposure of the recording medium is stopped.

Although the recording material layer is transparent with respect to the exposure light, the transmittance of the recording material layer varies as the photochemical reaction of the recording material due to exposure to light progresses. Generally, the transmittance is progressively increased as the photochemical reaction proceeds. For holograms with the same light interference information recorded to be optically equivalent to one another, their diffraction efficiencies must be the same. Since the diffraction efficiencies vary with the extent to which the photochemical reaction progresses, the diffraction efficiencies can be equalized by uniformizing the extent to which the photochemical reaction progresses at the time of completion of the exposure to light. With the diffraction efficiencies equalized, any number of optically equivalent holograms can be produced. Inasmuch as the extent to which the photochemical reaction progresses can be known from the change in the transmittance of the recording material layer, the extent to which the photochemical reaction progresses is detected by monitoring the change in this transmittance. The exposure of the recording medium to light is finished when the extent of the photochemical reaction reaches a certain level.

There are three ways of monitoring the change in the transmittance. The first process is to know the intensity B of light that has passed through the recording medium. This process is simplest, but useless when the exposure light itself varies, e.g., when the intensity of light emission from a laser beam source varies. If there is a change in the thickness of the recording material layer, the intensity of light that has passed through the recording material layer is caused to vary due to such a thickness change, and such a variation in the light intensity is interpreted as resulting from the progress of the photochemical reaction. This means that any slight change in the thickness of the recording material layer may cause an error. Consequently, in order for the first process to be relied upon, the thicknesses of the recording material layer and the substrate should be as uniform as possible, and the laser beam source used should be capable of emitting a stable amount of light.

According to the second process, a ratio between the intensity A of exposure light at the surface of the recording medium and the light intensity B, i.e., B/A or A/B, is used. This second process can automatically remove any influence of a change in the intensity of the exposure light. However, it cannot eliminate an error caused by a different thickness of the recording material layer. Therefore, the thickness of the recording material layer should be as uniform and constant as possible.

The third process employs a ratio between the light intensity B and the intensity Bo of exposure light that has passed through the recording medium immediately after the recording medium has started being exposed to the light, i.e., B/Bo or Bo/B. This process is capable of automatically removing any influence of a change in the thickness of the recording material layer, but is incapable of removing any influence of a change in the intensity of the exposure light. Since, however, the intensity of the exposure light varies over a period of time which is quite long as compared with the time for which the recording medium is exposed. As a consequence, the exposure light can be regarded as constant in intensity within the exposure time, and any influence due to a change in the intensity of the exposure light is negligible.

Because any influence of a change in the thickness of the recording material layer is also small, substantially optically equivalent holograms can be produced by using any one of the above first, second, and third processes. The produced holograms are most optically equivalent when the third process is employed, and are more optically equivalent when the second process is used than when the first process is used. Although slightly complex, use of B/ABo or ABo/B can prevent changes in the intensity of the exposure light and changes in the thickness of the recording material layer from affecting the diffraction efficiencies of produced holograms.

The values of A and B used in the second process may be variable with time; thus, A(t)/B(t) or B(t)/A(t) may be used where t is time. Alternatively, a value Ao of the light intensity at the surface of the recording medium immediately before or after exposure to light may be used as a constant in the second process.

According to the present invention, therefore, any number of optically equivalent holograms can be produced since the intensity of light which has passed through the recording medium and which is a function of the diffraction efficiency is used, and the amount of light to which the recording medium is exposed is controlled so that the diffraction efficiency will be constant.

The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of an operation sequence for a method according to an embodiment of the present invention;

FIG. 2 is a diagram, partly in block form, of a device for carrying out the method shown in FIG. 1;

FIG. 3 is a fragmentary plan view of a recording medium where a hologram is recorded;

FIG. 4 is a plan view of a mask; and

FIG. 5 is a flowchart of an operation sequence for a method according to another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a hologram recording device for carrying out a method according to the present invention.

The hologram recording device includes a laser beam source 10, a shutter 12, a mirror 14, a beam expander 16, a half-silvered mirror 20, a mirror 22 having few-percent transmittance, a mask 24, light detectors 26, 28, and a recording medium 50.

The recording medium 50 is in the form of a hologram disk which is circular in shape as shown in FIG. 3. The recording medium 50 has a substrate and a recording material layer coated thereon, which are transparent with respect to light emitted from the laser beam source 10. The recording medium 50 is seen from its back, i.e., the substrate, in FIG. 3, and includes an annular zone 50A in which holograms are recorded or photographed. The mask 24 is light-transmissive and includes a window 24A and a hole 24B as shown in FIG. 4. The mask 24 is fixedly mounted in the hologram recording device with the window 24A positioned in the annular zone 50A. Holograms are successively recorded on the recording medium 50. After one hologram is recorded, the recording medium 50 is turned about its center through a prescribed angle to bring an unexposed portion of the annular zone 50A over the window 24A of the mask 24 for recording a next hologram.

When the laser beam source 10 is energized and the shutter 12 is opened, the emitted laser light is reflected by the mirror 14 to be applied to the beam expander 16. Expanded light emitted from the beam expander 16 falls on the half-silvered mirror 20. A substantially half of the light falling on the mirror 20 is allowed to pass therethrough, and the rest is reflected thereby. The light that has passed through the half-silvered mirror 20 falls on the mirror 20 and is reflected thereby to fall on and pass through the recording medium 50 toward the light detector 26. The intensity of light detected by the light detector 26 is expressed as B. Since the mirror 22 has few-percent transmittance, a few percent of the light falling on the mirror 22 goes therethrough and falls on the light detector 28. The intensity of light detected by the light detector 28 is expressed as A, and may not necessarily be the intensity of light which falls directly on the recording medium.

A process of recording a hologram by using the device shown in FIG. 2 will be described with reference to FIGS. 1 and 2.

Before a hologram is recorded on the recording medium 50, a preset value K is entered by a keyboard 40 into an arithmetic/control circuit 38 which may comprise a CPU, a microcomputer, or the like.

The preset value K is a value that is to be reached by the ratio B/A when exposure of the recording medium 50 to light is stopped. The preset value K is determined in advance on experimental data that gives the relationship between the amount of light to which the recording medium is exposed and the diffraction efficiency thereof. Then, the shutter 12 is opened by the arithmetic/control circuit 38 to start exposing the recording medium 50 to the light.

The light intensities B, A detected by the light detectors 26, 28 are amplified by amplifiers 30, 32, respectively, and the amplified signals are converted by A/D converters 34, 36, respectively, to digital signals which are applied to the arithmetic/control circuit 38.

Based on the light intensities B, A that are successively applied as digital signals to the arithmetic/control circuit 38, the arithmetic/control circuit 38 successively computes the ratio B/A as a value S, compares the value S with the preset value K, and closes the shutter 12 when S=K. The hologram recording process is now completed. The above hologram recording process is repeated as many times as the number of holograms to be produced. The preset value K is entered only once when the series of hologram recording processes is started, and does not need to be entered in each hologram recording process.

Where the ratio B/Bo is used, the hologram recording process is carried out according to the flowchart of FIG. 5. According to this process, the light detector 26, the amplifier 32, and the A/D converter 36 are dispensed with, and the mirro 22 may be an ordinary mirror.

A preset value K which is to be reached by the ratio B/Bo when the exposure is finished is entered, and then the shutter 12 is opened. The intensity B of light detected at this moment is stored as Bo, and the light intensity B is thereafter detected successively. The ratio B/Bo is computed as a value S, which is compared with the preset value K. When S =K, the shutter 12 is closed to finish the exposure.

Where the intensity of light applied to the recording medium 50 and hence the light detector 26 is expressed as BI, and the recording material layer and the substrate (such as of glass) of the recording medium 50 have respective transmittances ε,η, then the following equation is established:

    B=BI·εη

The transmittance ε varies as the photochemical reaction of the recording medium progresses, and the transmittance η may be regarded as a constant with respect to the varying transmittance ε. Therefore, the transmittance η is applied to the arithmetic/control circuit 38, B/η=β is used instead of B, and β, A/β, or βo/β (βo=Bo/η) may be employed as a value to be compared with the preset value K.

In order to prevent light reflected by the back of the substrate from being recorded as noise, a reflection preventing film is sometimes coated as a strip on the back of the substrate in covering relation to the zone where holograms are to be recorded. This reflection preventing film may block light that would otherwise be applied to the light detector 26. To avoid this, light that has passed through the recording medium may be transmitted through the hole 24B (located out of the region in which the reflection preventing film is formed) of the mask 24, and detected by the light detector 26 for the detection of the light intensity A.

Although certain preferred embodiments have been shown and described, it should be understood that many changes and modifications may be made therein without departing from the scope of the appended claims. 

What is claimed is:
 1. A method of recording a hologram on a recording medium comprising a substrate and a recording material coated as a thin layer on the substrate, the recording medium being transparent or semitransparent with respect to light to which the recording medium is exposed, said method comprising the steps of:detecting the intensity A of the light at a surface of the recording medium and the intensity B of the light that has passed through the recording medium while the recording medium is being exposed to the light; converting said intensities A, B to digital signals; computing a value B/A from said digital signals; comparing said computed value with a preset value K; exposing the recording medium to the light until B/A=K; and stopping the exposure of the recording medium when K=B/A.
 2. A method of recording a hologram on a recording medium comprising of a substrate and a recording material coated as a thin layer on the substrate, the recording medium being transparent or semitransparent with respect to light to which the recording medium is exposed, said method comprising the steps of:detecting the intensity B of the light that has passed through the recording medium and the intensity Bo of the light that has passed through the recording medium immediately after the recording medium has started to be exposed to the light while the recording medium is being exposed to the light; converting said intensities B, Bo to digital signals; computing a value B/Bo from said digital signals; comparing said computed value with a preset value K; exposing the recording medium to the light until B/Bo=K; and stopping the exposure of the recording medium when K=B/Bo.
 3. A method of recording a hologram on a recording medium comprising of a substrate and a recording material coated as a thin layer on the substrate, the recording medium being transparent or semitransparent with respect to light to which the recording medium is exposed, said method comprising the steps of:establishing a value B/ABo or ABo/B where A is the intensity of the light at a surface of the recording medium, B is the intensity of the light that has passed through the recording medium, and Bo is the intensity of the light that has passed through the recording medium immediately after the recording medium has started to be exposed to the light; exposing the recording medium to the light until said established value is equalized to a preset value; and stopping the exposure of the recording medium when said established value is equalized to said preset value.
 4. A method of recording a hologram on a recording medium comprising a substrate and a recording material coated as a thin layer on the substrate, the recording medium being transparent or semitransparent with respect to light to which the recording medium is exposed, said method comprising the steps of:establishing a value β, A/B, or Bo/ηβ where A is the intensity of the light at a surface of the recording medium, B is the intensity of the light that has passed through the recording medium, Bo is the intensity of the light that has passed through the recording medium immediately after the recording medium has started to be exposed to the light, η is the transmittance of the substrate, and β=B/η; exposing the recording medium to the light until said established value is equalized to a preset value; and stopping the exposure of the recording medium when said established value is equalized to said preset value. 